1. Field of the Invention
This invention relates to the purification of wastewater. More particularly, this invention relates to the purification of wastewater using hypochlorous acid.
2. Description of the Prior Art
For many years chlorine has been recognized as a good disinfectant and in one form or another has been involved in the majority of systems designed to treat water. In almost all modern water or sewage treatment plants, for example, chlorine is used to reduce the number of bacteria from the final effluent before it is discharged from the system to a potable water distribution system or into a river or stream. A chlorination plant or system may also provide for removing color, correcting tastes, eliminating odor and suppressing other types of undesirable biological growths. Chlorine is also widely used in the treatment of industrial wastes and waste waters.
The amount of chlorine added to the water is referred to as the "dosage" and is usually expressed as milligrams per liter (mg/1) or parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand".
Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent. The amount of chlorine remaining in the water at the time of measurement is referred to as the "residual". Residual is therefore determined by the demand subtracted from the dosage. Inasmuch as chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acid is formed. The hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions as a function of the pH of the water. In either the hypochlorous or hypochlorite form, chlorine is called "free chlorine residual". Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is invariably the case with sewage, then additional compounds called chloramines are created. Chloramines may occur almost instantaneously, depending mainly on water pH. Though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual ". This combined chlorine residual has a much lower bacterial effect than free chlorine residual.
Domestic wastewater is typically high in ammonia, the ammonia resulting primarily from hydrolysis of urea. Almost all of the inorganic nitrogen formed in solutions that enter a waste treatment plant is normally in the least oxidized, ammonia form. In conventional secondary waste treatment, a portion of the ammonia will be completely nitrified to nitrite, some ammonia will be only partially nitrified to nitrite, and a portion will remain as ammonia.
When sufficiently high chlorine dosages are applied to waters containing ammonia, different reactions will occur, resulting in the destruction of the ammonia and the formation of free chlorine residual. Thus, for water containing a known amount of ammonia, if one starts with a chlorine dosage which is low, chloramines will be formed resulting in a combined chlorine residual whose bacterial effect is relatively weak.
As the dosage is raised, the amount of combined chlorine residual produced also increases, until a peak is reached when all of the free ammonia is used up in the formation of chloramine. As the dosage is elevated beyond the level at which the combined chlorine residual peaks, destruction of the chloramines, which are unstable, takes place until a breakpoint is reached indicating that chloramine destruction is at its maximum. At breakpoint, the first persistent appearance of free chlorine occurs. Thus, by using a chlorine dosage sufficient to attain the breakpoint state, one is able to get rid of virtually all ammonia and most of the chloramines.
Many applications exist for chlorine in wastewater treatment facilities, such as for odor control of raw sewage and the control of hydrogen sulfide in sewers, but its most universal application lies in wastewater treatment facilities for the terminal disinfection of the treated plant effluent just before the effluent is discharged.
The formation of compounds suspected of being carcinogenic as a result of the reaction of chlorine with hydrocarbons in wastewater is by no means the only unwanted side effect caused by the traditional disinfection process, for chlorine residuals in wastewater give rise to an environment that is toxic to aquatic organisms. Though chlorine is a highly effective biocide for undesirable organism, it is also deadly to fish and other forms of aquatic life and therefore, has a deleterious impact on fresh water eco-systems.
In general, wastewater disinfection practice has heretofore largely disregarded these unwanted side effects, for this practice focused on the two factors thought to be of greatest significance in attaining adequate disinfection; namely, the residual of the disinfectant and its contact time with the sewage. This practice has brought about the use of massive doses of chlorine disinfectant in long serpentine channel serving to prolong contact time. While this produced the desired degree of disinfection, it also aggravated unwanted side effects.
In order to obtain adequate disinfection with minimal unwanted side effects, the now recognized goal is to carry out rapid, intimate mixing of the chlorine solution with the wastewater stream in the shortest possible period.
In treating water, the equipment which supplies Cl.sub.2 gas to water operates at partial pressures (vacuum). At the vacuum levels currently being used the maximum solubility is about 5000 mg/l. The upper limit of solubility recommended by all chlorinator manufacturers is 3500 mg/l.
U.S. Pat. No. 4,693,832, issued Sep. 15, 1987 to M. M. Hurst, describes a method of preparing potable water by mixing into semi-finished water an aqueous solution of hypochlorous acid having a pH of between about 3 and about 6 in amounts which provide the water with a free chlorine residual of at least about 0.5 ppm. The aqueous solutions contain between 0.1 and 10 grams of HOCl per liter. The method employs dilute solutions of impure hypochlorous acid having high pH values. These HOCl solutions are stated to provide the water with free available chlorine residuals having improved stability. However, to obtain the required pH range it is necessary to supply a base to the hypochlorous acid solution or employ a method of preparation which will prepare hypochlorous acid solutions having the desired pH.